def avg(lst):
    a={}
    for key in lst:
        s=len(lst[key])
        b=0
        for i in range(s):
            b+=lst[key][i]
        a[key]="{:.2f}".format(b/s)
    return a
s={'小李':[77,54],'小张':[89,66,78,99],'小陈':[90],'小杨':[69,58,93]}
print('每个同学的平均成绩是:',avg(s))
